The present invention relates to recording and display apparatus. More particularly, it relates to improvements relating to line scan cathode ray tube recording apparatus.
It is known that in cathode ray tube devices, the phosphor coating on the inner surface of the tube face has the disadvantage that when it is driven hard, the phosphor exhibits a limited phosphor life. That is, after a determined length of time, the phosphor no longer responds to the electron beam stimulus to create an effective light image. The effective life of the phosphor is an inverse function of how hard the phosphor is driven. In cathode ray tube recording devices, the phosphor is excited with a high intensity electron beam in order to gain a high order of light emission whereby to effect a reasonable record. In the so-called line scan type recording tube, the electron beam is deflected substantially only in the X-direction. This causes the beam to traverse repeatedly over a very narrow area on the phosphor surface. It has been heretofore recognized that such traversing of a single small line across the face of the phosphor coating soon causes the phosphor to become ineffective. To alleviate this aspect of the problem, there has been disclosed in the above-identified U.S. Pat. No. 3,700,955, a technique for imparting a slight vertical shift to the scanning cathode ray beam between predetermined limits and at a relatively slow rate, to cause the beam to traverse across differing lines on the face of the CRT but still within a reasonable range to avoid distorting the value of the record.
In utilizing such apparatus in making analog records of variable signals, there has been superimposed upon the analog data periodic pulses of the cathode ray beam to cause the appearance of grid lines on the resultant chart. These grid lines are formed by periodically pulsing the unblanking circuit for the CRT at the same transverse spots for each successive scan of the scanning cathode ray beam. The repeated excitation of the same portions of the phosphor needed to generate such grid lines causes an early deterioration of the phosphor at those spots. In an effort to distribute the wear on those spots over a wider area, there has been provided in the past, a slow ramp bias to cause the grid lines to slowly drift from a fixed starting position. For example, during the course of the running of a full roll of chart paper, the grid lines would drift from a starting position to an end position approximately one-eighth of an inch to the right. While such a change is very slight it still introduces a measure of distortion in the resulting record.